If you get a letter to show up and pick up your PR card and you can not because you are outside Canada, you contact the CIC office that sent the letter and tell them that. Worst case, if you end up losing the chance of your first free PR card, you can apply for a PR card once you arrive in Canada. It will be treated like a renewal so you will have to wait around 3 months, you will need new photos for it and you will have to pay a $50 application fee.
Your visa office can not refuse you a travel document if you still meet the residency obligation. In order to meet the residency obligation, you must stay in Canada 2/5 years (730 days) which means not be outside Canada more than 3/5 years (1095 days). If you have been a PR for less than 3 years, it is impossible for you to be in breach of the residency requirements because even if you left right away after landing, you'd still have time to come back and stay 730 days so as long as you take care of this before that time, you have absolutely nothing to worry about.